The present invention relates to a method of exposing a lattice pattern onto a photo-resist film, and more particularly to a method of exposing a lattice pattern onto a photo-resist film by use of an optical exposure system.
Manufacturing processes for semiconductor devices my be classified into main three processes of process for forming films, process for etching to the films and lithography processes. A typical one of the lithography process is carried out as follows. An organic film so called to as a photo-resist film is formed over a substrate. An exposure system so called to as a stepper is used to irradiating a light or an X-ray through an exposure mask having a pattern of light shielding material onto the photo-resist film, so that either an exposed part or an unexposed part of the photo-resist film is selectively removed by a development, thereby to form the same pattern as the exposure mask pattern or scale-reduced pattern over the substrate. Other lithography process is to use an electron beam. An organic film so called to as a resist film is formed over a substrate. An electron beam is continuously irradiated onto the organic film with one stroke, so that either an exposed part or an unexposed part of the resist film is selectively removed by a development, thereby to form the same pattern as the exposure mask pattern or scale-reduced pattern over the substrate.
The exposure includes both irradiation of light or X-ray and irradiation of electron beam.
It is often required to form lattice patterns which comprise regular lattice alignments of fine patterns which are geometrically shaped. Particularly, two-dimensional periodically-structured photonic crystal is one of optical devices which need fine lattice patterns. Those lattice patterns are, for example, tetragonal lattices, hexagonal lattices, and honeycomb-structured lattices. FIG. 10 is a diagram illustrative of a fine tetragonal lattice of circles. FIG. 11 is a diagram illustrative of a fine hexagonal lattice of circles. FIG. 12 is a diagram illustrative of a fine honeycomb-structured lattice of circles.
It is most important for realizing those fine lattice pattern that possible fine photo-resist patterns are formed as accurately in size as possible.
In the semiconductor field, the most frequently used optical exposure system is the stepper which utilizes g-ray having a wavelength of 486 nanometers or i-ray having a wavelength of 356 nanometers. The exposure is carried out to irradiate those ray through the exposure mask having the same size pattern or an enlarged size pattern directly onto the photo-resist film for direct patterning to the photo-resist pattern. If those exposure systems are used, the minimum size of lattice point for possible patterning to the photo-resist depends upon the wavelength of the used light both when the photo-resist remains after development and when the photoresist is removed after development. As the wavelength of the used light is short, the minimum size of the lattice point of the photo-resist lattice pattern is also made fine. For example, it is easy for using the i-ray to minimize the size of the lattice point of the photo-resist lattice pattern rather than when the g-ray is used. If a further minimization of the size of the lattice point of the photo-resist lattice pattern under the condition of using the i-ray stepper, it is possible to use a super resolution technique such as a phase shift mask. If further more fine lattice pattern is required to be formed by use of the optical exposure system, it is possible to use a KrF excited dimer laser having a wavelength of 249 nanometers.
The above method of transferring the lattice pattern of the exposure mask onto the resist by use of g-ray, i-ray, and KrF excited dimer laser has the limitation to minimize the size of the lattice point. It is difficult for this method to form the above fine lattice patterns for photonic crystal. Either when the photo-resist remains after development or when the photo-resist is removed after development, the minimum size of the lattice point by use of g-ray is about 0.7 micrometers. The minimum size of the lattice point by use of i-ray is about 0.5 micrometers. The minimum size of the lattice point by use of KrF excited dimer laser is about 0.45 micrometers. When the i-ray is used together with the phase shift mask, the minimum size of the lattice point is about 0.45 micrometers. A phase shift mask for KrF excited dimer laser has not yet been developed due to technical difficulty. When the photonic crystals of cylindrically shaped which form hexagonal lattice as shown in FIG. 11 are applied, then the diameter of the cylindrically shaped lattice points is ranged about 0.25 micrometers through 0.4 micrometers, which are smaller than the above minimum size obtained by the optical exposure.
X-ray exposure and electron beam exposure have been known in the art to which the invention pertains. The wavelength of X-ray and the de-Broglie wavelength are sufficiently smaller than wavelengths of g-ray, i-ray and KrF excited dimer lasers. The minimum diameter of the circle-shaped lattice point formed by the X-ray exposure is 0.1 micrometer which may be applicable to the two-dimensional periodically-structured photonic crystal. The minimum diameter of the circle-shaped lattice point formed by the electron beam exposure is not more than 0.1 micrometer which also may be applicable to the two-dimensional periodically-structured photonic crystal.
The X-ray exposure has a problem with difficulty in forming the X-ray mask. The X-ray mask comprises a thin base layer of Si having a thickness of about 2 micrometers and a heavy metal pattern such as Au pattern having a thickness of about 1 micrometer which is formed on the thin base layer. The X-ray mask is so thin as being difficult to control or suppress the strain of the X-ray mask. The manufacturing cost for the X-ray mask is much higher than that of the optical exposure mask. As the X-ray source, a synchrotron radiation source is used, which is extremely large and expensive instrument as compared to the stepper used as the optical exposure system. The use of the X-ray exposure to form the fine lattice pattern causes the problem with the increased manufacturing cost.
On the other hand, the electron beam exposure has another problem with difficulty in exposure over a large area. One electron beam scanning is capable of exposure over not more than a few hundred micrometers square. If larger area is required to be exposed to the electron beam, then it is possible to divide the exposure-required large area into a plurality of small exposure areas, each of which has an area of not more than a few hundred micrometers square. The divided small exposure areas are discontinuously exposed to the electron beams. Namely, one of the divided small exposure areas is exposed to the electron beam scanning which is different from the electron beam scannings for the other divided small exposure areas. The next exposure of the electron beam to the other divided small exposure area is carried out after a stage on which a substrate has been moved, for which reason boundaries between the divided small exposure areas have discontinuations in electron beam exposures due to error in accuracy of movement of the stage. Accordingly, the boundaries between the divided small exposure areas have discontinuations in the lattice patterns due to the error in accuracy of movement of the stage and strain of the pattern. This discontinuation of the lattice pattern is serious problem because it is extremely important for the lattice patterns that lattice points are regularly arranged. Therefore, the electron beam exposure is actually applicable to form the lattice pattern over a few hundreds micrometers squares. The optical exposure is responsible for forming the lattice pattern over a larger area of not less than about ten millimeters squares. The maximum area on which the lattice pattern may be formed by the electron beam exposure is only {fraction (1/1000)} to {fraction (1/100)} of the area on which the lattice pattern may be formed by the optical exposure. 
The electron beam exposure is carried out by one-stroke irradiation of the electron beam, whilst the optical exposure is carried out by one-shot irradiation of the optical beam, for which reason the necessary time for the electron beam exposure is much longer than the necessary time for the optical exposure. The use of the electron beam exposure causes an extreme reduction in throughput of the semiconductor device having the fine lattice pattern, whereby the turn around time necessary for manufacturing the device is increased and also the manufacturing cost is also increased.
Further, the photo-resist for the electron beam exposure is lower in dry etching resistivity than the photo-resist for the optical exposure.
In the above circumstances, it had been required to develop a novel method of optical exposure to form a fine lattice pattern having a size-reduced lattice points, free from the above problem.
Accordingly, it is an object of the present invention to provide a novel method of optical exposure to form a fine lattice pattern having a size-reduced lattice points, free from the above problems.
It is a further object of the present invention to provide a novel method of method of optical exposure to form a fine lattice pattern with introduction of lattice defects into the lattice pattern.
The present invention provides a method of optical exposure to form a lattice pattern on a photo-resist, wherein at least two times of multiple exposure are carried out by use of different patterns.
The multiple exposures with the different patterns are carried out for forming a lattice pattern onto the photo-resist by utilizing the facts that the exposed part of the photo-resist has a high solubility to a development whilst the unexposed part of the photo-resist has an insolubility to a development. The multiple exposures are capable of higher exposure than the electron beam exposure. The multiple exposures are also capable of highest resolution limit of optical exposure method in forming the fine lattice pattern.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.